The E12's were the first 5 Series cars, so-called because they were the fifth new series after the V-8 and Isetta era.

The other four were the 02, 2500/2800/Bavaria, CS and the 3 Series.

BMW E12 5 Series, 1973
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The first 5 series was introduced in 1972 as a European only model. 

The 520 and 520i's were 4 door Sedans with carbureted and fuel injected four-cylinder engines.

The 525 appeared in 1973 as the first six-cylinder model. The 525 had 145 hp and its successor, the 528, had 164 hp.

In 1974, motivated by the worldwide fuel crisis, BMW introduced the frugal 90 hp, four cylinder 518.

The six cylinder 530i was introduced to the American market in 1974. Road & Track magazine made the 530i one of its Ten Best Cars. The 530i boasted a 176 hp engine, four-speed manual transmission, and a top end of 124 mph.

The early US models had several problems including cylinder head cracking, body rust, and ugly "safety" bumpers. Spoilers to the rescue!

Several changes were made in 1977 such as improved ventilation, increased sound insulation, electrically operated side mirrors, two-way sunroof, and ventilated front disc brakes.

Standard equipment included sport wheels, air conditioning, power windows and power steering. 

From 1975 to 1977, BMW produced a 528 Sedan with twin carburetors for the European market. In August 1977, the fuel injected 528i was introduced with 176 hp and a Bosch L-Jetronic induction system.

In 1978 the 528i was exported to the USA with a 3-way catalytic converter and a Bosch Lambda sensor to insure that all emissions standards were met.

And then in 1979 a miracle happened: the first M5 was built for Europe. The M5 had a six cylinder engine with 218 hp and a top speed of 225 kmh/140 mph. The birth of a legend!